We investigate deuterium atoms adsorbed on the surface of liquid helium in equilibrium with a vapor of atoms of the same species. These atoms are studied by a sensitive optical method based on spectroscopy at a wavelength of 122 nm, exciting the 1S-2P transition. We present a direct measurement of the adsorption energy of deuterium atoms on helium and show evidence for the existence of resonantly enhanced recombination of atoms residing on the surface to molecules.
We investigate deuterium atoms adsorbed on the surface of liquid helium in equilibrium with a vapor of atoms of the same species. These atoms are studied by a sensitive optical method based on spectroscopy at a wavelength of 122 nm, exciting the 1S-2P transition. We present a direct measurement of the adsorption energy of deuterium atoms on helium and show evidence for the existence of resonantly enhanced recombination of atoms residing on the surface to molecules.
We have optically detected hydrogen atoms adsorbed on the surface of liquid helium, a system relevant for the study of Bose degeneracy in two dimensions. The atoms are excited by 121.6 nm light and detected both in fluorescence and in absorption. The excitation spectrum shows a resonance that is much broader than that of a free atom, and shifted to lower frequencies. From the absorption signal we determine that we have reached a surface density corresponding to one atom per square de Broglie wavelength. We also use the hydrogen fluorescence to probe the thermal resistance between the free surface and the bulk of the liquid helium.
Current limitations on the density of stabilized spin-polarized deuterium (D\ensuremath{\downarrow}) to \ensuremath{\sim} 1014 atoms/cm3 are shown to arise from condensation on the 4He surface and subsequent recombination. A new technique for measuring the adsorption energy is employed to provide the first such measurement for D\ensuremath{\downarrow} on helium, yielding $\frac{{\ensuremath{\epsilon}}_{a}}{{k}_{B}}=2.5\ifmmode\pm\else\textpm\fi{}0.4$ K.
In this thesis we describe the results of two sets of experiments on spin-polarized atomic hydrogen at temperatures below 1 K using a 115 GHz heterodyne ESR spectrometer. First, we have made measurements of the rates of the two-body recombination processes in spin-polarized atomic hydrogen gas and spin-polarized atomic deuterium gas in a 41 kG field. In the second set of experiments, we examined the ESR absorption line associated with the spin-polarized atomic hydrogen atoms adsorbed on surfaces of liquid helium. In hydrogen, a comparision of the measured recombination rates in 41 kG with the corresponding rates measured in different magnetic fields showed that an additional recombination process is activate for the conditions of these experiments. We demonstrate that this is due to a resonant recombination process via the (v,J) = (14,4) level of molecular hydrogen. In spin-polarized deuterium, we found that the recombination rate are much larger than the analogous rates in hydrogen, and therefore that atomic deuterium gas is much less stable than the hydrogen system under comparable conditions. We report the first observation and study of doubly spin-polarized deuterium, a gas in which both the deuteron and electron spins are aligned. At temperatures near 0.1 K, the absorption line of doubly spin-polarized H atoms changes drastically as a peak appears on both sides of the peak associated with the bulk atoms. We have shown that these new peaks are due to the atoms which are adsorbed onto the surface of the liquid helium film that coats the walls of the microwave cavity. The geometry dependent non-zero average magnetic dipolar field due to aligned spins on a planar surface results in a shift of the position of the ESR absorption lines. The lineshapes of the side peaks are very unusual and are best described as ramp-like. Even after extensive attempts to explain this lineshape, we still do not know what mechanism is responsible.
We study three-body recombination in the gas of spin-polarized atomic hydrogen adsorbed on the surface of superfluid helium at temperatures from 45to130mK. The two-dimensional gas is thermally compressed to densities up to $\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ using the ``cold spot'' method which makes the three-body process the dominant decay channel in the system. We measure the loss rate and surface density of atoms directly and independently and observe the former to be proportional to the third power of the latter. The result for the surface three-body recombination rate constant at 4.6T, ${L}_{s}=2.0(7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{4}∕\mathrm{s}$, significantly reduces the discrepancy between the theory and earlier measurements where the surface density was inferred from the adsorption isotherm. We also measured the three-body rate constant on a 0.1% $^{3}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He}$ film and found ${L}_{s}=1.3(4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{4}∕\mathrm{s}$. This larger value is attributed to an increased delocalization of adsorbed hydrogen atoms in the direction normal to the surface.
The binding energy of a hydrogen atom to the surface of liquid 4He has been calculated to be at least 0.6K by Mantz and Edwards. This means that the properties of adsorbed spin-polarized hydrogen (H↑), a 2D Bose fluid, may soon be observed experimentally. Since 4He is used as a neutral coating for vessels to contain H↑ gas, adsorption onto the 4He surface may enhance the rate of recombination into H2 molecules. For given temperature and density in the gas, the amount of H↑ adsorbed depends strongly on the H↑-H↑ interaction on the 4He surface. We estimate the interaction in a simple approximation, and find that it limits the surface density to not more than 1014 cm-2. Nevertheless this density should be sufficient for a 2D superfluid transition to be observed, under conditions close to those achieved experimentally.
The contents of this thesis include several experiments and a theory, the unifying theme of which is resonant recombination of atomic hydrogen (H) and deuterium (D) at low temperatures. Measurements of the interaction of H and D with the liquid helium (l-⁴He) wall coating used in low-temperature experiments are also included. Atomic hydrogen and deuterium were studied by ESR in a field of 41 kG at temperatures below 1 K, in a second generation of experiments on an existing apparatus. The binding energy of H on l-⁴He was measured, by direct observation of the physisorbed atoms, to be E[sub B]= 1.00(5) K. Extensive meaurements of recombination rate constants were made. Resonant recombination of H via the (v, J) = (14,4) level of H₂ was observed and identified. Atomic deuterium was also observed to recombine in a resonant process. At low temperatures, resonant recombination is possible due to the predissociation of weakly bound molecular levels by the intra-atomic hyperfine interaction. The theory of resonant recombination of H and D due to hyperfine predissociation is developed in detail. In order to keep the development self-contained, it is presented as appendix D of this thesis. The theory is combined with our ESR data in order to predict the magnetic-field dependence of resonant recombination of D. It is shown that observation of the threshold magnetic fields for resonant recombination will permit accurate measurement of the dissociation energy of the (v, J) = (21,0) and perhaps (21,1) levels of the D₂ molecule. An apparatus was constructed to study atomic deuterium by hyperfine magnetic resonance at temperatures near 1 K. The D atoms were produced from D₂ of 99.65% isotopic purity by an RF discharge in a bulb coated with a film of superfluid l-⁴He. Magnetic resonance in a field of 39 G on the longitudinal β-δ transition at its minimum frequency of 309 MHz was used to observe the atoms. The experiment was designed with the capability of measuring the recombination rate as a function of magnetic field for fields up to 30 kG by moving the sample between the 309 MHz resonator and a superconducting solenoid. A number of preliminary experiments were made using this apparatus. It was discovered that the D atom density decays away exponentially in time. The decay rate was measured as a function of temperature and found to follow an Arrhenius (thermally-activated) form, with an activation energy of about 15 K. No second-order decay was seen, permitting an upper bound to be placed on the D-D recombination rate. The exponential decay of the D atom density was due to the penetration by the atoms of the l-⁴He film coating their container. Our measurement of the temperature dependence of the density decay rate gives the energy required to dissolve a D atom in l-⁴He, Es[sub s] ≃ 14 K. This is the first experimental determination of this quantity. The longitudinal relaxation time T₁ for the β-δ transition was found to be too short to be due to D-D spin-exchange. It was conjectured that the sample was contaminated with substantial amounts of H, spin-exchange in H-D collisions causing the short T₁. Measurements in an existing 1420 MHz apparatus showed directly, by magnetic resonance on the hydrogen zero-field transition, that H atoms were, in fact, present. By varying the H concentration, we were able to show that the decay of the D density by was not due to recombination of H with D. Several attempts to substantially reduce the H impurity have failed. The penetration of the l-⁴He film is undoubtedly one reason why the D is so susceptible to contamination by H, since the film is permeable to D but not to H. For use in interpreting these experiments, the effects of spin-exchange relaxation on hyperfine resonance in mixtures of H and D were calculated.